Lyme disease, caused by Borrelia burgdorferi, is the most prevalent arthropod-borne disease of humans in the United States and many other countries throughout Europe and Asia. Tick-borne relapsing fever, caused by Borrelia hermsii, is endemic in scattered foci throughout many regions of higher elevation in the western United States. The objectives of this project are to (1) use recombinant DNA techniques to express specific antigens of B. burgdorferi and B. hermsii to improve the serologic discrimination between these two diseases, (2) to acquire new, low passaged, infectious isolates of spirochetes, and (3) to examine adaptive molecular responses produced by B. burgdorferi and B. hermsii during infection in their respective tick vectors. This last objective also has relevance to the development of new serological antigens and vaccines. Our work to improve on the serodiagnosis of both Lyme disease and tick-borne relapsing fever has used recombinant DNA technology to clone genes of spirochetes that express proteins that induce specific and detectable antibody responses in humans infected with borreliae. We now have a considerable reference collection of Lyme disease spirochetes and no longer devote effort to isolate these spirochetes from natural sources. However, very few isolates of relapsing fever spirochetes exist, thus we are continuing to make every effort possible to establish new isolates through our collaborations with local, state, and private health laboratories. This work is done by receiving potentially infected material, such as ticks from relapsing fever case investigations, and using mice followed by culture media to isolate spirochetes in pure culture. Our studies examining how spirochetes adapt to their tick and mammalian hosts now receive most of our attention. For this, we maintain colonies of both Ixodes scapularis and Ornithodoros hermsi, the respective tick vectors of Lyme disease and relapsing fever spirochetes, and infect these ticks via a laboratory mouse - tick cycle. For this work, we utilize the infectious spirochetes acquired in our second goal cited above. Both species of ticks are infected with spirochetes, and then later examined by immunofluorescence staining for the distribution of spirochetes within ticks. Antisera and monoclonal antibodies specific to various outer surface proteins allow us to use fluorescence microscopy to determine how the surface of these bacteria change during tick feeding and as the bacteria are transmitted from tick to mammal and from mammal to tick.